1. Technical Field
The invention relates to storage and recovery of data on magnetic tape units and, more particularly, to application of array techniques to removable media data storage systems to improve data throughput and reliability. Still more particularly, the invention relates to management of and data organization on removable media volumes to support array operation.
2. Description of the Related Art
Redundant array, inexpensive disk (RAID) systems are well known for improving data rates and, with use of redundant data, storage reliability. RAID systems are implemented with a plurality of hard drive disk systems. The drives are typically synchronized to allow data to be divided among and written to the drives along parallel channels. This is called data striping. In theory, a RAID system is faster than single drive systems by a factor equal to the number of non-redundant data disk drives used in the array, because of the wider bandwidth made available by use of parallel channels.
Data striping in itself does not improve reliability, because there is increased exposure to failure of any one disk drive when an array of drives is in use. However, error correcting codes, parity information, or both, can be used to make a RAID system which is tolerant of failure of one or more disk drives within the array. In these systems one or more of the drives in the array is used strictly for storage of redundant data. These redundant drives may carry parity information, which when combined with an error signal generated using error correcting code on a failing non-redundant drive, or an error signal resulting from simple failure of a non-redundant drive, may be used to regenerate the data on the failing non-redundant drive. See, for example, U.S. Pat. No. 4,775,978 to Hartness. Alternatively a plurality of redundant drives may be used for error correcting code which may be used directly to recover missing data. See U.S. Pat. No. 4,722,085 to Flora et al.
Where magnetic tape units are used for archival backup of RAID systems it would be desirable to match the bandwidth of the RAID system into the magnetic tape units. Most RAID concepts are not inherently limited to disk drives and can be applied to a group of magnetic tape units to broaden bandwidth for write and read operations. However, direct application of RAID controller processes to arrays of magnetic tape units, or other devices using removable media, raises potential pitfalls which, if unaddressed, could result in complete loss of data files.
In a magnetic tape unit the storage media is virtually always removable and replaceable. Volumes, e.g. mountable tape cassettes, must be tracked and control units and accessors advised of correct volumes when data is accessed. Thus there is not an array of magnetic tape units as such, but rather an array of volumes available for mounting on the magnetic tape units.
It is characteristic of RAID systems that the storage media is nonremovable. This allows operation of the disk drive base RAID to be made transparent to a host. In other words, a RAID system appears to a computer to be a single disk drive. The volumes used with magnetic tape units cannot be hidden from a host computer and thus an array of volumes mounted on magnetic tape units cannot operate as a single logical device. Volumes must be tracked to assure correct mounting and storage, and design fail-safe procedures should be provided to recover from incorrect mounting of volumes. Some method of handling volume control information distinct from user data information must be provided. Handling media defects for tape volumes is more complicated than for hard disk drives in that the defect map for a given magnetic tape unit can change with each remount of volume on the unit. Finally, magnetic tape units are sequential access rather than direct access devices, raising issues relating to tracking associated blocks of data after striping.